Grits for sanding are deposited on the backing support either by gravity or in an electrostatic field in order that each individual grain shall be secured in an upright position, giving maximum cutting performance to the teeth. Figure 32 shows the disposition of the grains where A is the backing, B the basic adhesive coat, usually of hide glue, C the second adhesive coat of hide glue or synthetic resin and D the abrasive grain. The use of hide glue for the basic coat imparts a certain degree of flexibility to the sanding sheet, while the synthetic resin in the second coat anchors the grain in an upright position and does not soften under the heat generated by the cutting action. Generally speaking, grits anchored entirely by hide glues cut softer because the heat softens the glue, permitting the upright grain to lean over slightly, while the use of resin glues throughout gives a harder, fiercer cutting action, and will withstand the considerable degree of heat generated; therefore manufacturers use either one or a combination of both, according to the type of abrasive material and the purpose for which it is to be employed. Waterproof papers for wet sanding are anchored exclusively with synthetic resin glues.
The types of abrasive material available for wood finishing are as follows:
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